Protecting device against interfering electromagnetic radiation comprising EMI-gaskets

ABSTRACT

The object of the invention is a device and an elongated EMI gasket ( 20 ), made from an electroconductive wire, used for its EMI screening (electromagnetic interference). It is characteristic of the EMI gasket that it comprises a groove substantially parallel to the longitudinal direction of the EMI gasket for receiving a piece or its part ( 43 ) inside the EMI gasket ( 20 ). The piece to be protected against interfering electromagnetic radiation is EMI screened by pushing the EMI gasket ( 20 ) against the edge of the piece and by arranging contacts through the EMI gasket between the piece and some electroconductive surface that is close to it.

PRIORITY CLAIM

This is a national stage of application No. PCT/FI00/00570, filed onJun. 22, 2000. Priority is claimed on that application and on theApplication No. 991454, filed in Finland on Jun. 24, 1999.

FIELD OF THE INVENTION

The present invention relates to protecting devices against interferingelectromagnetic radiation.

BACKGROUND OF THE INVENTION

Electromagnetic radiation generated in a device may disturb either thedevice's own operation or the operation of some external device.Generally, the aim is to protect sensitive devices and interferingsources of electromagnetic radiation against radiation by encasing themin packages made from a conductive material and by sealing the packagesso tight that no interfering electromagnetic radiation can penetrate thepackage. This type of or similar protection of devices againstelectromagnetic radiation is called EMI (electromagnetic interference)shielding.

One problematic area in EMI shielding is the sealing of junctions andjoint surfaces comprised by devices, device cabinets and boxes. If thejoint surfaces are not properly sealed with EMI gaskets, interferingelectromagnetic radiation will quite easily pass through the joint. Thebest protection against interference is achieved when the joint surfacesare tightly sealed together galvanically. This means that resistancebetween the joint surfaces, so-called ‘junction resistance’, is as lowas possible. However, it is difficult and expensive to manufacture suchplane-like joint surfaces, where the surfaces are tightly attached toeach other in every place galvanically. Therefore, solutions in which agood contact between the joint surfaces is not formed in every place butat certain distances along the whole length of the joint, are used forsealing joint surfaces. When the distance between the contacts formed issufficiently short, electromagnetic radiation can no longer penetratethe joint in disturbing quantities. A sufficient contact distancedepends on the frequency of the interfering radiation and the requiredattenuation level. Mechanical properties and the available space alsoaffect the contact distance used. In connection with device cabinets andracks, a typical contact distance can be, e.g. 5-15 mm.

EMI sealing is required in various types of electric devices. Amongothers, EMI gaskets are used in device box and cabinet doors andapertures, as well as in partitions between different units insidedevice cabinets.

There are at least three types of gaskets that are most commonly usedfor EMI sealing. In one solution, a mantle is knitted from a conductivematerial around a resilient rubber compound or some other correspondingmaterial. The mantle is knitted from a very thin wire that acts as aconductive fabric. When placed in between joint surfaces, these types ofgaskets give an even contact but do not necessarily give a sufficientcontact for EMI shielding due to the large contact area. They do notpierce through the surface, which is slightly oxidised or greasy. Thesetypes of gaskets may shed short pieces of wire, which can cause a shortcircuit after being passed on to a printed board. Neither do they endurefriction and continuous wear.

In a second solution conductive particles are mixed inside a rubber-likesealing compound, the conductive particles forming a galvanic connectionbetween joint surfaces when the joint surfaces are pressed together.However, the electroconductivity of these types of gaskets does not comenear to that of, e.g. copper alloyed gaskets. Furthermore, theproperties of these types of gaskets may change as they age.

A third solution is provided by spring-like gaskets bent from sheetmetal. Their electroconductivity is good, but their manufacture isproblematic. The manufacture of spring-like sheet metal gaskets requiresexpensive perforating and bending tools. In addition, the edges of thegaskets are sharp, whereupon one may hurt one's hand on them, and thelength of the gaskets is limited to the length of the sheet used intheir manufacture, which normally is about 70 cm, in which case afull-length gasket must be assembled from several pieces.

The most significant disadvantage of a spring-like sheet metal gasketis, however, its susceptibility to being damaged due to its poor elasticproperties. The gasket has extremely accurate tolerance of compression.If joint surfaces are pressed together too little, the gasket placed inbetween them will leak, as it is called, i.e. let electromagneticradiation significantly through it. If again joint surfaces are pressedtoo much, a permanent deformation will take place in the gasket and itscompression force will no longer be sufficient. Also in this case, thejoint will begin to leak.

FIG. 1 illustrates an EMI gasket presented in the Patent PublicationU.S. Pat. No. 5,091,606, which comprises a helical spring made from acircular profiled wire and may comprise a layer made on top of thespring from a conductive and ductile material. When this type of gasketis placed in between the surfaces to be sealed and the surfaces arepressed against each other, a contact is formed between the surfaces.The gasket is intended for sealing shafts and other surfaces with acircular cross-section, and their circumferences. A disadvantage ofthese types of gaskets is a reasonably complex manufacturing process, aswell as the difficulty of fitting the gasket into small spaces. Inaddition, it is difficult to attach and get it stay in its place instructurally difficult joining points.

SUMMARY OF THE INVENTION

Now, EMI sealing has been invented, which is particularly well suited,e.g. for the EMI shielding of device cabinet partitions, shoulders,printed boards and other similar type of pieces and parts of the pieces.It is characteristic of an elongated EMI gasket, of the invention,manufactured from an electroconductive wire that the EMI gasketcomprises a groove substantially parallel to its longitudinal directionfor receiving a piece or a part of the piece inside the EMI gasket.

Correspondingly, it is characteristic of a device according to theinvention, the device comprising a first part and a second part, joinedtogether, and an elongated EMI gasket for getting said first part andsaid second part into contact and for preventing electromagneticinterference from penetrating a joining point between the first andsecond parts, that

the EMI gasket comprises a groove substantially parallel to thelongitudinal direction of the EMI gasket for receiving at least one ofsaid first and second parts inside the EMI gasket and that

the EMI gasket is in contact with both said first part and said secondpart for forming an electric contact through the EMI gasket between saidparts.

It is characteristic of a second device according to the invention, thedevice comprising a first part, a second part and a third part, joinedtogether, and in between them an elongated EMI gasket, made from anelectroconductive wire, for getting said parts into contact with eachother and for preventing electromagnetic interference from penetratingthe joining points between said parts, that the EMI gasket is in contactwith said first part and said second part, and that the EMI gasketcomprises a groove substantially parallel to the longitudinal directionof the EMI gasket for receiving said third part at least partly insidethe EMI gasket and that

said groove is bordered by a first edge, which is in contact with saidthird part for forming an electric contact between said first part; saidsecond part and said third part.

In accordance with the invention, the EMI gasket is made from aspring-like electroconductive wire that is bent in the appropriate shapedepending on the use, however, so that it comprises a groovesubstantially parallel to the longitudinal direction of the EMI gasket,from where a piece to be EMI sealed can at least partly penetrate insidethe EMI gasket. The piece to be protected against interferingelectromagnetic radiation is EMI shielded by pushing the EMI gasket tothe edge of the piece and by arranging contacts via the EMI gasketbetween the piece and some electroconductive surface that is close toit.

Other objects and features of the present invention will become apparentfrom the following detailed description considered in conjunction withthe accompanying drawings. It is to be understood, however, that thedrawings are intended solely for purposes of illustration and not as adefinition of the limits of the invention, for which reference should bemade to the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention will be explained in detail by referringto the enclosed drawings, in which

FIG. 1 shows an EMI gasket according to prior art;

FIGS. 2a-2 c show one way of EMI sealing a component package partitionaccording to the invention;

FIGS. 3a-3 b show the EMI sealing of a component package compartmentaccording to the invention;

FIGS. 4a-4 b show a second embodiment of EMI sealing according to theinvention;

FIGS. 5a-5 b show a third embodiment of EMI sealing according to theinvention; and

FIG. 6 shows a fourth embodiment of EMI sealing according to theinvention.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

FIG. 1 was described above in connection with the description of priorart. FIGS. 2a and 2 c are related to a first embodiment according to theinvention, in which a device cabinet partition is sealed EMI tight.Here, by the device cabinet is meant all types of packages used forprotecting devices, the external dimensions of which may typically varyfrom some tens of millimeters to a few meters. An EMI gasket 20 used forsealing can be made, e.g. by winding a sealing wire with a circularprofile in turn for a specific first length clockwise andcounter-clockwise around the longitudinal axis of the gasket 20. Alwayswhen the direction of winding changes, the sealing wire is allowed torun parallel to the longitudinal axis of the gasket 20 for a specificsecond length. In this way, it is possible to make the elongated helicalspring-like EMI gasket 20 according to the invention (FIG. 2a), thecross direction profile of which is open, preferably in the shape of acircular arc. The EMI gasket 20 comprises a groove parallel to thelongitudinal direction of the gasket for receiving an edge of the pieceto be EMI shielded inside the gasket 20. Alternatively, the crossdirection profile of an EMI gasket according to the invention can alsobe bent in other usable shapes, e.g. in the shape of a square where oneside of the square is at least partly open.

Typically, a device cabinet (FIG. 2b) to be sealed is made from anelectroconductive material, such as a die-casting aluminium alloy, andit comprises a first part 21 comprising a shoulder 23, which can be,e.g. a partition of a cabinet, and a second part 22, which is typicallynot shouldered. Said second part 22 can be, e.g. a part of the body ofthe device cabinet or a printed board in the cabinet.

Typically, the partition 23 separates from each other in an electronicdevice two blocks that have different functions, which neverthelessbelong to the same body structure, i.e. the same unit. EMI sealing isrequired between the blocks in order that the different blocks would notinterfere with each other, e.g. due to their radio frequency (RF)radiation. Typical such blocks are, e.g. low-frequency andhigh-frequency parts, analog and digital parts, as well as processunits. Said devices can be, e.g. devices in a mobile communicationnetwork. They can be located, e.g. in a base transceiver station of themobile communication network.

The penetration of interfering electromagnetic radiation from one sideof the shoulder 23 to the other side is prevented by placing the EMIgasket 20 that comprises a groove parallel to the longitudinal directionof the gasket between the shoulder 23 and said second part 22 so thatthe shoulder 23 is pressed partly inside the gasket 20, and by pressingsaid first part 21 and said second part 22 of the cabinet against eachother. This being the case, the edges that border said groove come intostrong contact with the shoulder 23 and the part of the gasket that isopposite to the opening of the groove with the surface of the secondpart 22 of the device cabinet, whereupon a compression is produced onthe EMI gasket 20 (FIG. 2c).

The elastic forces that act in the compressed gasket 20 tend to returnthe gasket into its original shape, whereupon the flexible gasket 20presses tightly against the second part 22 of the cabinet and theshoulder 23 comprised by the first part 21. This being the case, firmelectric contacts are formed between said first and second parts, joinedtogether, at close, regular distances preventing electromagneticradiation from penetrating the joint.

Typically, partitions wind inside the cabinet bordering different-sizedcompartments. In order that the structure to be sealed would be EMItight, the EMI gasket 20 must run along the whole length of the joint,and wind accordingly.

The gasket 20 can be made from several different materials, e.g. fromalloyed copper metal, stainless steel or other corresponding material.The gasket's electrical and mechanical properties, as well asmanufacturability, can be affected by the selection of the sealingmaterial. The diameter of the wire used for the manufacture of thegasket 20 may vary, but it is typically approximately 0.3-5 mm. Thediameter of the gasket 20 can be, e.g. 2-40 mm. The pitch of thread canbe, e.g. 1-15 mm. The sealing wire is bent in the required shape by aspring-making machine designed for bending wire material.

FIGS. 3a and 3 b illustrate how a whole device cabinet compartment isEMI sealed from its environment. FIG. 3a shows a device cabinetcomprising three compartments made from an electroconductive material.The body of the device cabinet comprises two different halves: a firsthalf 32 and a second half 31, which are joined together, e.g. withscrews 33. With the help of a shouldered partition 34, the body bordersinside the device cabinet three compartments, which have printed boards35 attached to the body 31 of the device cabinet with screws, with thehelp of elevation pins 37. On the printed board, in the centre cabinetcompartment, there is an interfering source 36 of electromagneticradiation. According to the invention, electromagnetic radiation,produced in the centre compartment of the device cabinet, is preventedfrom penetrating into the other compartments by EMI sealing according tothe first embodiment of the invention, in which the EMI gaskets 20presented in the first embodiment are installed compressed on thepartitions 34 of the device cabinet.

FIG. 3b differs from the case presented in FIG. 3a in that thereseparate printed boards 35 of the compartments are replaced by a singlecommon printed board 38, which extends to all three compartments. Thisbeing the case, the EMI gaskets press the conductive surface on theprinted board 38, which must be in ground potential for achieving EMItightness, i.e. in the same potential as the body of the cabinet.

The EMI gasket 20 presented in the first embodiment is well suited forbeing installed on the edge of different types of boards, e.g. printedboards 43 (FIG. 4a). FIG. 4b illustrates the second embodiment accordingto the invention, in which a device cabinet comprising the printed board43 is EMI sealed. The body of the device cabinet comprises a first part41 and a second part 42, which can be joined together, e.g. with screws44. Said first part 41 comprises elevation pins 45 with the help ofwhich the printed board 43 can be attached to the body of the cabinetwith screws 46. On the surfaces of the printed boards 43 that pointtowards said first part 41 and said second part 42 of the cabinet, thereis preferably located as ground plane an electroconductive area, whichcan be made, e.g. from electroconductive metal film; foil. On thesurfaces of the printed board, there can be electric components 47.

For achieving EMI tightness, the EMI gasket 20 is first pushed on to theedge of the printed board 43 and the printed board with the gasket isplaced in the cabinet so that part of the EMI gasket 20 leans againstthe corner of said first part 41 of the cabinet. After this, the secondpart 42 of the cabinet is matched together with said first part 41 andsaid parts 41, 42 are pressed together, e.g. with the screws 44. Thisbeing the case, the EMI gasket, which is in a recess produced betweensaid parts 41, 42, compresses and the gasket 20 is pressed tightlyagainst the surfaces of said first part 41 and said second part 42 ofthe cabinet. The parts of the EMI gasket 20 that border the groove thatis parallel to the longitudinal direction of the gasket 20 press againstan electroconductive area located on the surface of the printed board43. This being the case, electric contacts are formed at close, regulardistances between said cabinet parts 41, 42 and the ground plane of theprinted board 43, whereupon the penetration of interferingelectromagnetic radiation into and out of the cabinet is prevented. Thepenetration of radiation inside the cabinet from one side of the printedboard to the other side is also prevented.

An EMI gasket 50 can also be made so that a recess 51, adapted for theedge of the printed board, is bent in the part of the gasket that isopposite to the opening of the gasket 50, whereupon the gasket followsbetter the edge of the printed board 43 (FIG. 5a). The edges of saidrecess 51 preferably touch the edging parts of the printed board 43.This being the case, the printed board 43 attaches more effectively tothe gasket 50 and there are more contact points between the gasket andthe board. One advantage of this structure is also that if, for example,the gasket is pressed from one side of the printed board 43 more thanfrom the other side or if it is only pressed from one side of theprinted board 43, the gasket 50 will keep its shape well and not tend totwist in relation to the edge of the printed board 43.

It is often appropriate, for example, due to space limitations that aplurality of printed boards are located in the same device cabinet. Inthis case, besides the EMI shielding of the whole cabinet, it must alsobe seen to it that the printed boards, typically stacked on top of eachother in the cabinet, do not interfere with each other due toelectromagnetic radiation. FIG. 5b illustrates the third embodimentaccording to the invention, in which three printed boards 53-55, stackedon top of each other in a device cabinet, are EMI insulated from eachother. The body of the device cabinet 52 comprises two parts betweenwhich the middle one 54 of the printed boards reaches. The printedboards 53-55 are attached to each other through intermediate bushings56. Typically, there are electric circuit components 57 on at least oneside of the printed board. For EMI insulating the printed boards 53-55from each other, Emi gaskets 50 that follow the edge of the printedboard are placed compressed on the edge of the outermost printed boards53, 55 according to the invention. When pressing together the body partsof the cabinet 52, the EMI gaskets 50 form contacts, at close, regulardistances with conductive areas located on the surface of the printedboards 53-55 and with the body of the cabinet 52 dividing the cabinet 52into different kinds of spaces that are EMI insulated from each other.

The parts of the body of the device cabinet 52 can also be shaped sothat the recess presented in connection with the second embodiment ofthe invention is produced in the cabinet, whereupon the EMI gasket 50can also be installed on the edge of said middle printed board 54, whichEMI gasket 50 when the body parts are pressed together presses in saidrecess against the body parts of the cabinet preventing interferingelectromagnetic radiation from penetrating into the device cabinet orout of the device cabinet through the joint between the body parts ofthe cabinet 52.

FIG. 6 shows the fourth embodiment according to the invention, in whichan EMI gasket 60 that operates as presented above is installed outside adevice cabinet to ensure the EMI tightness of the cabinet. The cabinetcomprises a specific first part 61 and a specific second part 62, whichare attached together by pressing the EMI gasket 60 in tension on top ofa shoulder 63, which shoulder 63 is produced when said parts 61, 62 ofthe cabinet with suitably shaped edging parts are placed against eachother. In this case, said EMI gasket 60, which may have a cross-sectionin the shape of, e.g. a rectangle opened on one of its sides, contactswith said parts 61, 62 of the device cabinet preventing interferingelectromagnetic radiation from penetrating the joint between the cabinetparts. No traditionally used attachment methods, such as screws, areneeded for attaching the device cabinet parts 61, 62, but instead theEMI gasket 60 placed in tension on top of the shoulder 63 takes care ofthe attachment.

The ease of attachment of an EMI gasket according to the invention isone essential advantage of the invention. No gluing is required for theattachment. An edge or a shoulder of a plate, against which the EMIgasket can be pushed, is sufficient for the attachment. The versatilityof the EMI gasket is an advantage. An EMI gasket according to theinvention can be easily replaced if this is necessary, e.g. with an EMIgasket, which has a diameter of a different size.

This paper presents the implementation and embodiments of the inventionwith the help of examples. A person skilled in the art will appreciatethat the present invention is not restricted to details of theembodiments presented above and that the invention can also beimplemented in another form without deviating from the characteristicsof the invention. The presented embodiments should be regarded asillustrative but not restricting. Thus, the possibilities ofimplementing and using the invention are only restricted by the enclosedclaims, and the various options of implementing the invention asdetermined by the claims, including the equivalent implementations, alsobelong to the scope of the invention.

What is claimed is:
 1. An elongated EMI (electromagnetic interference)gasket (20, 50, 60), characterised by the EMI gasket (20, 50, 60) beingmade from an electroconductive wire bent in a spring-like shape suchthat the EMI-gasket comprises: a longitudinal portion formed by saidwire, the portion being parallel to the longitudinal direction of theEMI gasket, and a groove substantially parallel to the longitudinaldirection of the EMI gasket, the groove being adapted to receive a pieceor a part of the piece (23, 34, 43, 53, 55, 63) inside the EMI gasket(20, 50, 60).
 2. An EMI gasket according to claim 1, characterised inthat the EMI gasket (20, 50, 60) is arranged to touch said piece or thepart of the piece (23, 34, 43, 53, 55, 63) with said longitudinalportion.
 3. An EMI gasket (20, 50, 60) according to claim 1,characterised by the EMI gasket being manufactured by bending anelectroconductive wire so that it forms an elongated element thetransversal profile of which being open.
 4. An EMI gasket (20, 50)according to claim 3, characterised in that its transversal profilecomprises a circular arc.
 5. An EMI gasket (60) according to claim 3,characterised in that its transversal profile is a rectangle open on oneof its sides.
 6. An EMI gasket (50) according to claim 1, characterisedin that said groove further comprises a recess (51) opposite to theopening of the groove, walls of the recess being adapted to touch saidpiece or its part (23, 34, 43, 53, 55, 63).
 7. An EMI gasket (20, 50)according to claim 1, characterised in that at least one of the edgesthat border said groove is adapted to touch said piece or its part (23,34, 43, 53, 55, 63).
 8. An EMI gasket (20, 50) according to claim 1,characterised in that said piece or its part (23, 34, 43, 53, 55) is apartition of a device cabinet or a circuit board.
 9. A device thatcomprises a first part (21, 32, 61) and a second part (22, 31, 62)joined together and an elongated EMI gasket (20, 60) for getting saidfirst part (21, 23, 32, 61) and said second part (22, 31, 62) intocontact and for preventing electromagnetic interference from passingthrough the joining point between the first and the second part,characterised by the EMI gasket (20, 60) being made from anelectroconductive wire bent in a spring-like shape such that the EMIgasket (20, 60) comprises: a longitudinal portion formed by said wire,the portion being parallel to the longitudinal direction of the EMIgasket, and a groove substantially parallel to the longitudinaldirection of the EMI gasket, the groove being adapted to receive atleast one of said first and second parts (23, 34, 43, 53, 55, 63) insidethe EMI gasket, and wherein: the EMI gasket (20, 60) is in contact withboth said first part and said second part for forming an electricalcontact via the EMI gasket between said parts (21, 22; 32, 31; 61, 62).10. A device according to claim 9, characterised in that the EMI gasket(20) is in between said first part (21, 32) and said second part (22,31).
 11. A device according to claim 9, characterised in that the secondpart (22, 31) of the device touches an external surface of the EMIgasket (20) and the first part (21, 32) of the device is partly insidethe EMI gasket (20) and said groove is bordered by a first edge, whichis in contact with said first part (21, 32), whereupon the EMI gasketforms a contact between said first part (21, 32) and said second part(22, 31) of the device.
 12. A device according to claim 9, characterisedin that both the first part (61) and the second part (62) of the deviceare partly inside the EMI gasket (60) and said groove is bordered by afirst edge and a second edge, which edges are in contact with said firstpart (61) and said second part (62) of the device respectively,whereupon a contact between said first and second parts is formed, viathe EMI gasket (60), externally around the joining point between saidfirst and second parts (61, 62).
 13. A device that comprises a firstpart (41), a second part (42) and a third part (43), joined together,and in between them an elongated EMI gasket (20) made from a conductivewire for getting said parts into contact with each other and forpreventing electromagnetic interference from passing through the joiningpoint between said parts (41, 42, 43), characterised by the EMI gasket(20) being in contact with said first part (41) and said second part(42) and by the EMI gasket (20) being made from an electroconductivewire bent in a spring-like shape such that the EMI gasket (20)comprises: a longitudinal portion formed by said wire, the portion beingparallel to the longitudinal direction of the EMI gasket, and a groovesubstantially parallel to the longitudinal direction of the EMI gasket,the groove being adapted to receive said third part (43) at least partlyinside the EMI gasket (20), and wherein: said groove is bordered by afirst edge, which is in contact with said third part (43) to form anelectrical contact between said first part (41), said second part (42)and said third part (43).